In several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites (MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film-growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities.

Reducing Detrimental Defects for High-Performance Metal Halide Perovskite Solar Cells.

Perovskite solar cells (PSCs) have become a promising photovoltaic (PV) technology, where the evolution of the electron-selective layers (ESLs), an integral part of any PV device, has played a distinctive role to their progress. To date, the mesoporous titanium dioxide (TiO2 )/compact TiO2 stack has been among the most used ESLs in state-of-the-art PSCs. However, this material requires high-temperature sintering and may induce hysteresis under operational conditions, raising concerns about its use toward commercialization. Recently, tin oxide (SnO2 ) has emerged as an attractive alternative ESL, thanks to its wide bandgap, high optical transmission, high carrier mobility, suitable band alignment with perovskites, and decent chemical stability. Additionally, its low-temperature processability enables compatibility with temperature-sensitive substrates, and thus flexible devices and tandem solar cells. Here, the notable developments of SnO2 as a perovskite-relevant ESL are reviewed with emphasis placed on the various fabrication methods and interfacial passivation routes toward champion solar cells with high stability. Further, a techno-economic analysis of SnO2 materials for large-scale deployment, together with a processing-toxicology assessment, is presented. Finally, a perspective on how SnO2 materials can be instrumental in successful large-scale module and perovskite-based tandem solar cell manufacturing is provided.

Tin Oxide Electron‐Selective Layers for Efficient, Stable, and Scalable Perovskite Solar Cells

Carbon-based electrodes have been widely applied in perovskite solar cells (PSCs) because of their chemical inertness and compatibility with up-scalable techniques, signifying their solid potential for mass-production. The material scarcity and complexity of metal ore extraction further highlights that conventionally used noble metal electrodes cannot represent a sustainable option for back-contact in PSCs, while cells with carbon-based electrodes represent an excellent solution to these problems. However, their power conversion efficiencies (PCEs) still lag behind the traditionally processed cells with metal electrodes, resulting in a considerable efficiency gap. To overcome this issue, we propose the use of low-temperature carbon-based (LTCB) electrodes, which offer several key advantages in comparison to mesoscopic high-temperature treated ones: (1) larger choice of selective layers, (2) applicability to all perovskite crystallization methods, (3) compatibility with flexible substrates and (4) faster deposition process. In this review, we analyze numerous techniques to formulate the LTCB-paste and ways to deposit it on the cell stack which have been developed in order to improve the interfacial contact and electrode conductivity. Besides describing the current state-of-the-art perovskite solar cells with LTCB-electrodes, the most promising strategies to enhance the PCE of such photovoltaic (PV) devices are discussed. Overall, we emphasize that PSCs with a LTCB-electrode combine high device stability, low manufacturing costs and low environmental impact, while having options for pushing the efficiency of carbon-based PSCs closer to the record-breaking cells, all of which are vital in order to fulfill the true potential of perovskite PV.

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Low-temperature carbon-based electrodes in perovskite solar cells

Organic–inorganic hybrid perovskite solar cells (PSCs) have made unprecedented progress in the past ten years, the power conversion efficiency of which increased from 3.8% in 2009 to 25.5% in 2020. The choice of hole transport layers (HTLs) is a key factor for achieving efficient and stable PSCs. Recently, 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) has been proven to be the most suitable small molecule HTL material in n–i–p PSCs. However, the conductivity and hole mobility of spiro-OMeTAD are dependent on the dopants. Bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) and 4-tert-butylpyridine (TBP) have become the standard HTL dopants to improve the charge transport properties of n–i–p PSCs. Both of these two dopants are effective but have a negative impact on the device stability (induced by ion migration, hygroscopicity, and corrosiveness). In response, lots of efforts have been devoted to the development of alternative and new dopants. In this review, the influence of different doping strategies and corresponding doping mechanisms on device performance and stability was mainly discussed. The stability issues of doped spiro-OMeTAD based PSCs were also described. Finally, we proposed some possible future research goals for dopants based on these existing results to obtain long-lasting and high-efficiency PSCs.

Strategies of modifying spiro-OMeTAD materials for perovskite solar cells: a review

Organic-inorganic halide perovskite solar cells (PSCs) have drawn tremendous attention owing to their remarkable photovoltaic performance and simple preparation process. However, conventional wet-chemical synthesis methods inevitably create defects both in the bulk and at the interfaces of perovskites, leading to recombination of charge carriers and reduced stability. Herein, a bilateral interface modification to perovskites by doping room-temperature synthesized CsPbBr3 nanocrystals (CN) is reported. The ultrafast transient absorption measurement reveals that CN effectively suppresses the defect at the SnO2 /perovskite interface and boosts the interfacial electron transport. Meanwhile, the in situ Kelvin probe force microscopy and contact potential difference characterizations verify that the CN within the upper part of the perovskites enhances the built-in electric field, facilitating oriented migration of the carriers within the perovskite. Combining the superiorities of CN modifiers on both sides, the bilaterally modified CH3 NH3 PbI3 -based planar PSCs exhibit optimal power conversion efficiency exceeding 20% and improved device stability.

CsPbBr3 Nanocrystal Induced Bilateral Interface Modification for Efficient Planar Perovskite Solar Cells.

Metal halide perovskite solar cells are emerging candidates for next‐generation thin‐film photovoltaic devices with the potential for extremely low fabrication cost and high power conversion efficiency. Perovskite solar cells have demonstrated a rapid development in device performance over the last decade, from an initial 3.81% to a most recently certified 24.2%, though the challenges of long‐term stability and lead toxicity still remain. Carbon materials, ranging from zero‐dimensional carbon quantum dots to three‐dimensional carbon black materials, are promising candidates for the enhancement of both efficiency and stability of perovskite solar cells, offering unique advantages for incorporation into various device architectures. In this review article, we present a concise overview of important and exciting advancements of perovskite solar cells that incorporate different dimensions of carbon material in their device architectures in an effort to simultaneously improve device performance and long‐term stability. We also discuss the major advantages and potential challenges of each technique that has been developed in the most recent work. Finally, we outline the future opportunities toward more efficient and stable perovskite solar cells utilizing carbon materials.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/eem2.12035

Carbon Materials in Perovskite Solar Cells: Prospects and Future Challenges

Defect Engineering toward Highly Efficient and Stable Perovskite Solar Cells

Interface engineering is an effective means to enhance the performance of thin‐film devices, such as perovskite solar cells (PSCs). Herein, a conjugated polyelectrolyte, poly[(9,9‐bis(3′‐((N,N‐dimethyl)‐N‐ethyl‐ammonium)‐propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)]di‐iodide (PFN‐I), is used at the interfaces between the hole transport layer (HTL)/perovskite and perovskite/electron transport layer simultaneously, to enhance the device power conversion efficiency (PCE) and stability. The fabricated PSCs with an inverted planar heterojunction structure show improved open‐circuit voltage (Voc), short‐circuit current density (Jsc), and fill factor, resulting in PCEs up to 20.56%. The devices maintain over 80% of their initial PCEs after 800 h of exposure to a relative humidity 35–55% at room temperature. All of these improvements are attributed to the functional PFN‐I layers as they provide favorable interface contact and defect reduction.

Tailoring Perovskite Adjacent Interfaces by Conjugated Polyelectrolyte for Stable and Efficient Solar Cells

Vertically aligned carbon nanotubes (VACNTs) present an exciting avenue for nanoelectronics due to their pre-determined orientation and exceptional transport capabilities along the tube length, with the potential to employ in a variety of optoelectronic applications. However, growth of VACNTs using conventional chemical vapour deposition (CVD) methods requires elevated temperatures (>720°C) and therefore, the suitability of commonly used transparent conductive oxide (TCO) glasses, such as fluorine-tin oxide (FTO) and indium-tin oxide (ITO), as the substrates for nanotube growth are limited by their temperature-sensitive nature. Here, the successful growth of multi-walled VACNTs directly onto commonly used TCO glasses, FTO and ITO, using the photo-thermal chemical vapour deposition (PTCVD) growth method is reported. The benefit of reflection, within the infrared region, of the TCO substrate and the effect of surface roughness on the growth of VACNTs is investigated. The application of VACNTs on ITO in inverted planar perovskite solar cells is investigated, which show superior charge transfer, larger grain sizes in the perovskite film, and a champion device efficiency approaching 16%.

Direct Growth of Vertically Aligned Carbon Nanotubes onto Transparent Conductive Oxide Glass for Enhanced Charge Extraction in Perovskite Solar Cells

Over the last two decades, emerging photovoltaic (PV) solar cells including dye-sensitized solar cells, organic solar cells and perovskite solar cells have made significant progress, enabling the generation of clean energy with higher power conversion efficiency and lower fabrication cost. During the advancement of these emerging PV technologies, nanocarbons (fullerenes, carbon nanotubes, and graphene) have played a central role in boosting the performance and long-term stability of devices, in conjunction with reducing the production cost of the devices. In this chapter, we briefly overview the concepts and structural evolution of the emerging solar cells and outline the challenges faced by these novel PV techniques toward their commercialization. We then summarize the structures and excellent properties (optical, electrical, thermal, mechanical, catalytic, etc.) of nanocarbons, followed by rationalization of the exciting achievements in recent years regarding the applications of nanocarbons at various length scales in emerging PV technologies. Finally, we discuss the prospective routes toward more efficient and stable solar cells that could compete with the silicon PVs which currently dominate the commercial market.

Victoria Ferguson

Papers

Carbon Materials in Perovskite Solar Cells: Prospects and Future Challenges

Defect Engineering toward Highly Efficient and Stable Perovskite Solar Cells

Tailoring Perovskite Adjacent Interfaces by Conjugated Polyelectrolyte for Stable and Efficient Solar Cells

Direct Growth of Vertically Aligned Carbon Nanotubes onto Transparent Conductive Oxide Glass for Enhanced Charge Extraction in Perovskite Solar Cells

Nanocarbons for emerging photovoltaic applications